Secondary batteries are utilized in various fields, for example, electronic devices such as cellular phones and personal computers, vehicles such as hybrid vehicles and electric vehicles, and others. Each of such secondary batteries includes a positive electrode plate(s), a negative electrode plate(s), and electrolyte. Separators are commonly provided between the positive electrode plate(s) and the negative electrode plate(s) for their insulation from each other.
The shapes of those batteries are, for example, cylindrical, rectangular, and so on. Electrode bodies used in those batteries include a wound-type electrode body in which a positive electrode plate and a negative electrode plate are laminated and wound in spiral form and a flat-laminated-type electrode body in which positive electrode plates and negative electrode plates are laminated flatly. For current collection from those laminated-type electrode bodies, the following method is conceived. A positive electrode core member is placed to protrude in one direction and its protruding portion is joined to a positive current collecting plate. Similarly, a negative electrode core member is placed to protrude in the other direction and its protruding portion is joined to a negative current collecting plate.
This joining is conducted by welding or the like, thereby providing mechanical connection and electrical connection. Such joined portions need to have joint strength and mechanical strength. In case the joined portions are partially peeled or broken away or in case the joined portions or their surroundings are broken down, such defective sites could not be fixed and lose the electric connection itself. This loss of electric connection disables current collection therefrom. The joined portions also need to have low electrical resistance, for the reason that energy loss becomes large as the electrical resistance is high.
Under the above circumstances, a technique to improve the mechanical strength of the joined portions has been developed. For example, Patent Document 1 discloses a secondary battery including a wound electrode body in which an end portion of the protruding portion of the positive electrode core member or negative electrode core member is slightly curved or bent, forming a burr. It is disclosed that this bun contributes to preventing the end portion of the protruding portion from becoming buckled or bent by pressure applied during welding, so that welding failures are avoided.
On the other hand, a technique to reduce electrical resistance has been developed. For example, Patent Document 2 discloses a cylindrical storage battery in which a positive current smoothing plate is welded to an end portion of a positive electrode core member and a negative current smoothing plate is welded to an end portion of a negative electrode core member. It is disclosed that those smoothing plates contribute to reducing internal resistance of the secondary battery and thus preventing sudden drop of voltage even when a large amount of current is supplied to the secondary battery.